Calculations of projected future changes in atmospheric composition rely on
a number of additional factors. For example, atmospheric models require forecasts
of future emissions if realistic predictions of the future atmosphere are to
be made. However, accurate forecasts of future demand are not possible. In this
report, the future is explored on the basis of scenarios. Scenarios should not
be interpreted as forecasts but as tools to explore a range of future outcomes.

To assess the possible future impact of aviation, plausible scenarios for other
changes in the future composition of the atmosphere are required-in particular
for background CO2, NOx, SO2, CO, and hydrocarbon emissions. The IS92 scenarios
used here were originally described in IPCC (1992). Six scenarios of future
greenhouse gas and aerosol precursor emissions (IS92a-f) were developed, based
on assumptions concerning population and economic growth, land use, technological
changes, energy availability, and fuel mix during the period 1990 to 2100. Through
an understanding of the global carbon cycle and atmospheric chemistry, these
emissions can be used to project atmospheric concentrations of greenhouse gases
and aerosols. In addition, scenarios for future atmospheric chlorine and bromine
abundances have been calculated by assuming that the Montreal Protocol and its
Amendments and Adjustments will be followed and effective. The IS92 scenarios
are recognized to be imperfect (for instance, they assume that no regulatory
interventions will be made, so they have been outdated by the UNFCCC process).
The underlying assumptions, strengths, and weaknesses were assessed in IPCC
(1995). The IS92a scenario is a mid-range emissions scenario and is used to
describe future non-aviation emissions in the calculations presented in this
report.

Box 1-3. Time Scales in Aviation and the Atmosphere

Understanding the time scales of the processes involved is important
in assessing the impact aviation can have on the atmosphere now and in
the future. It takes many years for a new aircraft design to progress
from the drawing board into service. Once aircraft are operative, their
emissions remain in the atmosphere for periods ranging from days to centuries,
with some climatic effects felt on even longer time scales. Furthermore,
although new technologies would have an immediate effect on emissions
from new aircraft, any impact on the global abundance of short-lived atmospheric
constituents would be limited by the rate of introduction of the new technology
into the global fleet. A rough idea of the various time scales involved
is provided. The processes that remove trace species from the atmosphere
can be chemical (e.g., the oxidation of methane), physical (e.g., in rain
or by dry deposition onto land or sea), or biological (e.g., the uptake
of CO2 by plants). The rate of each process typically varies with season
and location in the atmosphere. These rates can be combined to produce
a rough estimate of how long each constituent remains in the atmosphere.
A constituent with a short lifetime responds quickly to any change in
emissions. A trace species with a long lifetime responds slowly to a change
in emissions. The atmospheric effects of H2O, NOx, SOxO, and soot are all
relatively short-lived. Broadly speaking, the tropospheric lifetimes of
these constituents are a couple of weeks or less; that of any ozone produced
by NOx is a month or so. The stratospheric time scales involved are longer
but are all well under a decade. By contrast, emissions of carbon dioxide
affect the atmosphere for a long time (about 100 years), with little difference
for emissions into the stratosphere or troposphere. The main factors affecting
how quickly new aircraft are introduced are technological feasibility,
certification, and commercial viability. Typically, new technology is
likely to be a decade in its gestation, although this time scale may be
reduced if there are significant market opportunities. The project launch
of a new aircraft type by an airframe manufacturer is normally concurrent
with the launch of new engines supplied by competing manufacturers. Development
of the engine culminates in airworthiness and emissions certification,
usually 3-5 years later-but the time scale for entry into service is dictated
by the airframe manufacturer and its customer airlines. Once the engine
has achieved airworthiness certification, it is installed on the airframe,
and the aircraft typically then takes another year to complete the airworthiness
and noise certification process before initial deliveries are made to
customers. With commercial airlines, individual aircraft will operate
for 25 years or more in revenue service. A good product, including its
derivatives, will have a substantial production period (possibly 25 years
or longer); therefore, the overall time scale between introduction into
service of an aircraft type and withdrawal from service may exceed 50
years. Development of the infrastructure for air transportation (airports,
air traffic control, etc.) can take years or even decades. This development
is driven by overall increased demand for air transportation, both for
passengers and freight. It is limited by the availability of financial
resources and local environmental concerns about noise and increased ground
traffic around new or expanded airports.

The future growth of aviation will depend heavily on factors such as economic
growth (at global and regional levels), the demand for travel (in an age of
rapid advances in information technology), the development of infrastructure
to support air travel and available flight technology, and the availability
and cost of fuel. Increases in demand will not translate directly into increases
in emissions. Changes in engine efficiency, airplane design (size and shape),
and operational practice are all expected to lead to more efficient use of fuel
because there are strong commercial reasons for airlines and other operators
to keep fuel costs down.

The scenarios used in this report have been developed using models of passenger
demand on a regional and global basis that assume future economic growth rates
as found in the IS92 scenarios (particularly IS92a). In all cases, it is assumed
that infrastructure (e.g., airports) and technology will be developed so that
this growth is not constrained. Different aircraft types and fuel use are included,
so CO2 and H2O emissions-which depend solely on the amount of fuel burned-can
be calculated directly. SO2 emissions are estimated simply by assuming what
the sulfur composition in the fuel will be. Emissions of NOx, CO, and hydrocarbons
depend strongly on combustor technology-particularly the mixing of fuel and
air in the combustion chamber, as well as temperature and pressure. These emissions
are estimated using semi-empirical relationships between in-flight fuel flow
and emissions of NOx, CO, and hydrocarbons derived from ICAO engine certifications
together with determined flight patterns. Emissions of all of these compounds
for global air traffic are produced on a 3-D grid (latitude, longitude, altitude)
that can be used in global atmospheric models. Little equivalent information
regarding emissions exists for particulates other than total sulfur emissions.

In this report, future aviation emissions and their effects are assessed at
two different times in the future-2015 and 2050. The technological assumptions
in aviation demand models are relatively well determined for 2015, the first
year in which future impacts are assessed. Time scales for the development of
new aircraft types and technologies are too long for any radically different
option to become available and enter service to any significant extent by 2015
(see Box 1-3). The most significant uncertainties
relate to underlying economic growth and available aviation infrastructure,
which will be critical in determining how many aircraft are flying in 2015 and
what the relative numbers of each type will be (i.e., the fleet mix).

By 2050-the second year for which future impacts are assessed-many more technological
options could be introduced, and uncertainties about what will happen are much
larger. For example, a second generation of high-speed civil transport (HSCT)
aircraft could be operational in significant numbers (many more than the current
fleet of 13 Concordes) by the middle of the next century; these aircraft may
well replace some of the subsonic market. This development would be important
from an atmospheric perspective because HSCT emissions are released at significantly
higher altitudes than those of subsonic aircraft. The scenarios used here thus
cover a wider range of possibilities than for 2015. As for 2015, the calculated
demand is based on the economic growth rates in the IS92 scenarios, and allowance
is made for differential regional growth. Chapter 9 contains
a description of the methodologies used to produce a range of scenarios of emissions
from aviation. These scenarios assume idealized operational practices (i.e.,
direct routing, optimum flight profiles, and no delays for the assumed fleets).
They therefore represent minimum fuel use and emissions.